Conventionally, a number of devices for improving a directional stability and responsiveness on the turning of the vehicle are proposed. For example, in the Japanese Unexamined Patent Publication No. 4-193684, a four-wheel steering device of a vehicle, which accomplishes an ideal transient responsiveness property on the steering of the front wheels, is disclosed. The prior four-wheel steering device of the vehicle comprises front-wheel turning angle control means for controlling the turning angles of the front wheels, depending on the running condition of the vehicle and rear-wheel turning angle control means for controlling the turning angles of the rear wheels, depending on the running condition of the vehicle. Further, in the prior four-wheel steering device of the vehicle, the front-wheel turning angle control means controls the turning angles of the front wheels to accomplish the front-wheel turning angles given by a control transfer function expressing a Laplace-transformed value of the front-wheel turning angle corresponding to the Laplace-transformed value of the front-wheel steering input, while the rear-wheel turning angle control means controls the turning angles of the rear wheels to accomplish the rear-wheel turning angles given by a control transfer function expressing the Laplace-transformed value of the rear-wheel turning angle corresponding to the Laplace-transformed value of the front-wheel steering input.
Further, for example, in the Japanese Unexamined Utility Model Publication No. 62-145884, an actual turning angle control device of a vehicle is disclosed. The prior actual turning angle control device of the vehicle comprises transmission property changing means for variably setting a transmission property set to determine a target value of a motion condition amount on the steering of the vehicle, depending on an input for adjusting the steering property of the vehicle. Then, in this prior actual turning angle control device of the vehicle, the target value of the motion condition amount of the vehicle is changed by variably setting the transmission property by the transmission property changing means by the driver, and the actual turning angles of the wheels can be controlled such that the target value can be accomplished and the steering property of the vehicle can be changed.
Furthermore, for example, in the Japanese Unexamined Patent Publication No. 2008-189200, a steering device of a vehicle for improving the responsiveness and the convergence of the behavior of the vehicle in response to the steering operation is disclosed. This prior steering device of the vehicle is adapted to set a target turning angle on the basis of the sum of the proportional term proportional to the steering angle and the differential term proportional to the steering angle rate. Further, in this prior steering device of the vehicle, in case that the steering angle is negative, the proportional term is corrected such that it becomes a small value, compared with the case that the steering angle is positive.